1. Field of the Invention
The present invention relates to a process for stabilizing the magnetic properties of a ferromagnetic powder. More particularly, the present invention relates to a process for improving variation with time, and stability to pressure and mechanical force of the magnetic properties of iron oxide (MO.sub.x, x=1.36 to 1.47) which has a high coercive force (Hc) due to doping with Co. Further, the present invention relates to a process for stabilizing a magnetic recording member using the above ferromagnetic powder.
2. Description of the Prior Art
The production of high density magnetic recording members requires magnetic materials having a high coercive force and squareness ratio (ratio of residual magnetic flux to saturated or maximum magnetic flux, Br/Bm), and having a high magnetic flux density.
In increasing the coercive force of iron oxide based magnetic powders, the incorporation of cobalt is effective, and this method is described in, for example, U.S. Pat. No. 3,117,933, Japanese Patent Publication No. 6538/1966, Japanese Patent Publication No. 6113/1967 corresponding to U.S. Pat. No. 3,047,505, Japanese Patent Publication No. 15759/1973, Japanese Patent Publication No. 10994/1973 corresponding to British Pat. No. 1,318,579, and U.S. Pat. No. 3,671,435.
Since cobalt is relatively expensive, from a commercial standpoint the high coercive force preferably is obtained by the use of a small amount of cobalt, and furthermore, the amount of cobalt used preferably is small since cobalt also causes various disadvantages, that is, cobalt decreases the magnetic flux density as an impurity and increases the lack of stability of magnetic materials to mechanical force.
It is well known that partially reduced Co-containing maghemite (.gamma.-Fe.sub.2 O.sub.3) has increased coercive force due to the use of a small amount of cobalt.
This method is described in U.S. Pat. No. 3,573,980 and Japanese Patent Application (OPI) No. 51296/1973 (corresponding to U.S. Pat. No. 3,748,270), etc.
When a Co-containing iron oxide, particularly a magnetic material produced by doping cobalt onto an iron oxide having an oxidation ratio between those of magnetite (Fe.sub.3 O.sub.4) and maghemite (.gamma.-Fe.sub.2 O.sub.3), is allowed to stand after a heat-treatment at high temperatures, various changes with time, such as an increase in coercive force with time, an increase of print-through signals when a magnetic tape produced using the magnetic material is woundup and stored, and a decrease in magnetization due to mechanical friction and pressure are observed.
In overcoming the above undesirable properties, particularly the decrease in magnetization due to mechanical friction and pressure, Japanese patent application No. 28423/1974 (OPI) No. 119997/1975 (corresponding to U.S. patent application Ser. No. 557,431, filed Mar. 11, 1975) discloses the subjecting of magnetic materials to a heat-treatment at temperatures of from about 40.degree. to 100.degree. C, and the obtaining of an oxidation ratio of about 30 to 80%, as defined by the following relationship (I) ##EQU1## wherein y is the amount of a divalent metal ions contained in a magnetic iron oxide (atom %) other than divalent iron ions, and R is the ratio of the content of divalent iron ions to the total content of iron ions forming the magnetic iron oxide. When y in the relationship (I) is O, that is, when no divalent metal is present other than iron ions, MO.sub.x is FeO.sub.x and the relationship (I) becomes as follows: EQU Oxidation Ratio (%) = 100(6x-8) (II)
the present invention further improves this method.
The variation with time of the coercive force at room temperature (about 20.degree.-30.degree. C) of an iron oxide having an oxidation ratio between those of Co-containing magnetite and .gamma.-Fe.sub.2 O.sub.3 is described in Umeki et al, Variation with Time in Coercive Force of Cobalt-containing Iron Oxide Particles, Autumn Congress Abstract (1973), pp. 133 to 135 (October, 1971), published by Funtai Funmatsu Yakin Kyokai, and this variation is a serious problem in magnetic powders.
Maghemite (.gamma.-Fe.sub.2 O.sub.3) and magnetite (Fe.sub.3 O.sub.4) can be used as the iron oxide, but Berthollide iron oxide (FeO.sub.x where x is the oxidation degree) (which is disclosed in Japanese Patent Publication Nos. 5,009/64, 10,307/64 and 39,639/73) is especially effective presumably because diffusion of Co into such an oxide is easy. Berthollide iron oxides (FeO.sub.x) having an oxidation degree, as defined by the above-described relationship (I), of about 15 to 90% are especially effective.
In the above-described Berthollide iron oxide, x ranges from more than 1.33 to less than 1.50, preferably 1.36 to 1.49. Berthollide iron oxide is produced by following process.
a. A reduction of maghemite (FeO.sub.x, x = 1.50) to magnetite (FeO.sub.x, x = 1.33) where the reduction is stopped when 1.33 &lt;x&lt;1.50 as set forth in Japanese Patent Publication Nos. 5,009/64 and 39,639/73. PA1 b. An oxidation process of magnetite to maghemite where the oxidation is stopped when 1.33 &lt;x&lt;1.50 as described in Japanese Patent Publication Nos. 5,009/64 and 10,307/64.
Berthollide iron oxide produced by processes as described above is an iron oxide having an oxidation degree between that of magnetite (Fe.sub.3 O.sub.4 :FeO.sub.x, x = 1.33) and the maghemite (.gamma.-Fe.sub.2 O.sub.3 :FeO.sub.x, x = 1.50), and having a very high coercive force, decreased electrical resistance, and improved dispersibility in an organic solvent. Then, the print-through effect of a recorded signal to another magnetic recording layer is decreased. On the other hand, maghemite and magnetite can be used in combination, and the characteristics thereof are similar but not superior to those of the above-described Berthollide iron oxide.